Salmonellae are facultative intracellular pathogens that cause disease in humans and animals. Infections by these organisms result in a spectrum of diseases including enteric (typhoid) fever, gastroenteritis and bacteremias. These infections are more common and severe in infants and the elderly, as well as immunosuppressed individuals, including those with AIDS. Non-typhoidal Salmonella infections are often associated with contaminated food, including frequent egg-related outbreaks of Salmonella enteritidis. Salmonella typhimurium infection of mice causes a similar illness to, and is a model for, Salmonella typhi infection of humans. The overall objectives of this work are to better understand the induction of pathogenic bacterial gene expression in response to eukaryotic cell environments, as well as how bacteria utilize regulatory networks induced within these environments to avoid host defense mechanisms. S. typhimurium is able to survive within host phagocytic cells, a major site of production of antimicrobial peptides (AP). These cationic, broadly cytotoxic peptides are a major factor of the innate immune system. S. typhimurium has developed mechanisms of resistance to these peptides, and resistance is controlled by the in vivo induced two-component regulatory systems PhoP-PhoQ and PmrA-PmrB. PmrA-PmrB activation results in LPS modification and greatly reduced binding of the peptides to the surface of the organisms; however, the PmrA-PmrB regulated effectors and the mechanism(s) by which LPS modification occurs are unknown. The goal of this proposal is to characterize the PmrA-PmrB regulon, which includes identification and characterization of regulated genes whose products affect LPS structure and subsequent antimicrobial peptide resistance. These goals will be accomplished by (1) Genetic and functional characterization of the pmrF locus, a putative six gene operon necessary for AP resistance and modification of LPS with aminoarabinose; (2) Identification and characterization of additional PmrA-PmrB regulated genes necessary for AP resistance and LPS modification; (3) Definition of the role of PmrA-PmrB mediated LPS modification in S. typhimurium virulence and survival within professional phagocytes; and (4) Definition of cis-acting sequences necessary for PmrA binding to PmrA-PmrB regulated gene promoters. It is hoped that these studies will increase our understanding of the molecular basis of Salmonella pathogenesis.